The conventional process for demetalizing hydrocarbon oil generally utilizes an electrically desalting process in which a demetalizing agent, a demulsifier and water are mixed with hydrocarbon oil in a desired proportion, and the obtained mixture is then fed into a primary desalting tank and separated sufficiently into a primary desalted water and primary purified oil; the desalted primary water is discharged, and the purified primary oil from the primary desalting tank is mixed with the mixed solution of the demetalizing agent, the demulsifier and water proportionally; the resultant mixture is then fed into the secondary desalting tank to separate into oil and water under a predetermined electrical field, the isolated oil being the purified secondary oil and the secondary desalted water being either discharged or fed back to the primary desalting tank. Although the prior processes have the advantages of effective demetalization of hydrocarbon oil and easiness of operation, there are disadvantages such as insufficient utilization of the discharged water, severe pollution to the environment, waste of the demetalizing agent, demulsifier and water, and thereby high production cost of hydrocarbon oil per ton.